1. Field of the Invention
The present invention generally relates to multilayer ceramic substrates and a method for forming the same, and more particularly, relates to the metalizing of via holes using a dry method rather than the conventional metallic paste methods.
2. Description of the Related Art
Ceramic packages for supporting semiconductor devices and the like conventionally include a ceramic substrate with printed conductive circuits connected to the semiconductor device, to input/output pins or to other connections joined to boards or the like. While many techniques are known for forming such ceramic substrates, one of the most popular procedures for such fabrication involves the casting of what is termed a ceramic greensheet. The fabrication process involves personalizing ceramic greensheets, stacking, laminating, and then fusing them together (sintering) under elevated temperatures to form a substrate. This method of producing such multilayer ceramic (hereafter MLC) substrates for semiconductor packaging and other electronics applications is well known as illustrated in Herron et al. U.S. Pat. No. 4,234,367, Hetherington et al. U.S. Pat. No. 4,302,625, and Rellick U.S. Pat. No. 4,799,984, the disclosures of which are incorporated by reference herein.
In this multilayer ceramic (MLC) substrate technology, ceramic greensheets of ceramic powder (held together in sheet form by a suitable aqueous-based or organic-based binder) are punched to form via holes in a predefined pattern using a via punch tool.
The via holes are subsequently filled with a conductive paste and metallurgy lines are formed on the surface of the greensheet by screening or extrusion printing. The conductive paste is made of a suitable metallic material which will densify in a manner similar to the ceramic during the subsequent sintering process. The metallized sheets are stacked, laminated and fired in an appropriate atmosphere to form a monolithic MLC substrate with a complex internal circuitry.
The process of extrusion or screen printing to fill punched via holes with conductive paste can have several drawbacks. For example, in screen printing, a separate two mask/two pass operation is required. The first mask/pass fills the vias, and the second mask/pass prints an x-y circuit pattern. This two mask/two pass process is expensive. Alternatively, if extrusion printing is used, the vias can be filled and the x-y pattern printed simultaneously (e.g., in a single step). However, with the extrusion printing process, the same paste must be used to fill the via and create the x-y surface pattern. This is not always desirable. The above wet processes can also distort the punched ceramic greensheet as the solvents and oils in the metallic paste are absorbed into the greensheet. After screening, the sheets are dried in an oven. Additionally, these wet processes can require large amounts of chemicals to clean paste residue from the masks.
In the above processing, defects can arise due to screened paste bridging between adjacent lines or due to the absence of conductive paste in an intended contiguous line. With advancements in semiconductor technology driving toward increased total connections and decreased spacing between features, the fabrication of screen printed or extrusion printed conductors for subsequent cofiring can lead to reduced yields or the inability to effectively fabricate microelectronic components cost effectively. Additionally, as feature sizes become smaller, the desire for increased current carrying cross section becomes increasingly important to meet the needs of increasing power and higher frequency. However, such structures must also be fabricated with controlled impedance (matched to a chip of 50 to 55 ohms is typical). Also, it is critical to control noise by improving feature size uniformity and reduce inductance by using thinner dielectrics. For lamination and cofiring, the existing process can provide challenges such as having a non-uniform density of material across a green laminate. This can cause non-uniformity in sintering which can lead to non-flat packages or other defects.
Therefore, there is a need for a new process of placing metal features in such ceramic greensheets which avoids the expense of the conventional systems discussed above and which increases yield.
It is, therefore, an object of the present invention to provide a structure and method for filling an opening in a ceramic greensheet which includes positioning a sheet of conductive metal above the ceramic greensheet and punching the metal sheet into the opening in the ceramic greensheet. The components of the metal sheet and the ceramic greensheet have similar shrinkage characteristics when subjected to a subsequent sintering process.
The ceramic greensheet could be a ceramic insulator and the metal sheet could be a powder metal in an organic/polymer matrix. The metal sheet preferably has a thickness less than that of the ceramic greensheet. In one embodiment, the metal sheet is repeatedly punched into the ceramic greensheet until the opening is full. The distinct layers of the metal sheet elements punched into the ceramic greensheet can all be of one material composition or can be of varying mixtures of metal, ceramics, and organics. The distinct layers can form a capacitor or can comprise a series of graduated material features (e.g., stepped gradient via, inverted via, etc.) within the opening.
The metal sheet can be formed by a process that includes mixing metal containing powders with an organic binder and solvent to form a slurry and casting the slurry into sheets or other techniques where metal particles can be incorporated into an organic matrix.
Another embodiment of the invention is a process for forming a multi-layer capacitive structure in a substrate which includes punching a first material into an opening in the substrate and punching a second material different than the first material into the opening above the first material, such that the first material and the second material remain in the opening. This process could include punching additional material layers having different compositions or combinations of metal conductors plus fillers into the opening.
Yet another embodiment of the invention is a process for forming a multilayer ceramic substrate which includes positioning a sheet above a ceramic substrate and punching the sheet into an opening in the ceramic substrate, wherein the sheet and the ceramic substrate have similar shrinkage characteristics when subjected to a subsequent sintering process.